[Decline of infant and child mortality in the Dominican Republic: an exploratory investigation on changes in the structure and composition of socio-demographic determinants during the mortality transition]. / Descenso de la mortalidad infantil en la Republica Dominicana: una investigacion exploratoria sobre cambios en la estructura y composicion de los determinantes socio-demograficos durante la transicion de la mortalidad.

PIP: Results of the Dominican Republic's four most important fertility surveys were used to analyze how relations between selected sociodemographic factors and infant mortality have changed in recent decades, and to evaluate the contribution of changes in the distribution of these factors to mortality decline. The empirical analysis was carried out in four parts, the first of which analyzed trends in infant mortality in the Dominican Republic between 1960 and 1990. The second part established the importance of each determinant in each period studied and used logistic regression to analyze changes in the structure of the relationship between the variables and infant mortality. The third section analyzed changes in the relative weight of the categories of determinants during the study period. The final section standardized the risk of death in 1964-69 using the composition of births in 1985-90 and vice versa to evaluate the influence of each determinant in the infant mortality decline. The number of study variables was limited to six by lack of comparability in the four fertility surveys. The six variables were region and zone of residence, education and age of the mother, and birth order and interval. The results of this research did not indicate any clear pattern of change in the structure of relations between determinants and infant mortality. It appears plausible that there are various routes toward the transition leading to low mortality. In the Dominican Republic, maternal education was most important from 1964 to 1969, when infant mortality was high, and became relatively less important as a determinant in subsequent years. The changing relationship between maternal education and infant mortality suggests that there are no generalized patterns but rather the weight of each variable differs from one context to another and depends on the influence of other socioeconomic, medical, and demographic determinants.^ieng

[Spermatogonia types in tepezcuintle (Cuniculus paca). Ultrastructural study]. / Tipos de espermatogonios en "tepezcuintle" (Cuniculus paca). Estudio ultraestructural.

Ultrastructural studies of different types of spermatogonia by thin section techniques and transmission electron microscopy showed that Cuniculus paca has two types of Type A spermatogonia (A-O and A-1). Samples were taken at different stages of the cycle in the seminiferous tubule epithelium presented a similar pattern as those of other rodents. Type B spermatogonia are similar to those of the rat, the difference being that the heterochromatic zones are larger and more numerous. Both characteristics differentiate them from preleptotenic spermatocytes.

[Ultrastructure of the mitotic nuclei in Leishmania mexicana ssp. Tridimensional reconstruction of the mitotic spindle and dense plaques]. / Ultraestructura del núcleo mitótico en Leishmania mexicana ssp. Reconstrucción tridimensional del huso mitótico y placas densas.

The ultrastructure of mitotic nuclei of the promastigote Leishmania mexicana ssp. was studied by serial thin sections and three-dimensional reconstructions of each divisional stage. At the beginning of nuclear division (equatorial stage), a set of six dense plaques located about the equatorial region of the nucleus and a microtubular spindle develops in the two opposing poles of the nucleus (two sets of polar microtubules). The microtubular mitotic spindle is entirely intranuclear with the nuclear membrane persisting through mitosis. The polar spindle consists of a discrete bundle of about 50 microtubules and the equatorial spindle is formed by about 100 microtubules. The spindle may contain several continuous microtubules, but no microtubular organizing centres were observed in association with the spindle. The plaques and hemiplaques are associated with microtubular bundles; some of the spindle microtubules converge on kinetochore-like plaques. It is suggested that the spindle has a special significance in the physiology of mitosis. The two sets of hemiplaques may guide the separation of the daughter genomes. At the beginning of the elongational stage the mitotic plaques split into halves and each set of half-plaques migrates to one pole. It is concluded that the dense plaques play a kinetochore-like role and thus Leishmania mexicana ssp. may have six chromosomal units. Mitotic events of this species are essentially similar to those of Trypanosoma cruzi.

[Ultrastructure of spermatozoa of rats (Rattus norvegicus). Use of 3 technics in transmission electron microscopy]. / Ultraestructura del espermatozoide de rata (Ratus norvegicus). Aplicación de tres técnicas para microscopía electrónica de transmisión.

Ultrastructures spermatozoa taken from the seminiferous tubules of rat testes (Rattus norvegicus) were observed under transmission electron microscopy, using the techniques of ultrathin sections, microspreading specimens and replicas. The heads of isolated spermatozoa were of homogeneously high electron density, and had a slightly curved end; in longitudinal sections these zones were composed of a compact homogeneous DNA, covered by a nuclear envelop, cell membrane and small amounts of acrosomal material. The middle piece contained the modified centrioles in the junction zone and flagella microtubules. In transverse sections as well as in replicas, this region consisted of 10 pairs of microtubules and 9 dense fibers surrounded circularly by the mitochondria (mitochondrial sheath).

[Ultrastructural study in the epididymis of vasectomized hamsters (Mesocricetus auratus)]. / Estudio ultraestructural en epidídimo de hamster vasectomizado (Mesocricetus auratus).

Electron microscopy studies on the epididymal epithelium and lumen contents of hamster between 3 and 14 months after vasectomy, showed ultrastructural changes in the clear cells and spermatozoids within the lumen. These changes on the clear cells were as follows: accumulation of electron dense bodies of irregular forms distributed between luminal and basal zones of clear cells, presence of large vacuoles of low electron density and a loss of stereocilia; flattening of luminal border and emission of projections toward the lumen. Spermatozoid population was low within the lumen and is accompanied by large amounts of detritic material. Most of the gametes were destroyed. A variety of abnormal forms of spermatids were observed in advanced maturation stage.

[Ultrastructural study of the vasectomized hamster testis (Mesocricetus auratus)]. / Estudio ultraestructural en testículo de hamster vasectomizado (Mesocricetus auratus).

The ultrastructure of adult hamster (Mesocricetus auratus) testis was studied by means of light and electron microscopes during fourteen months after bilateral vasectomy. In all the vasectomized animals there was marked degeneration of the semniiferous tubules as well as reduction of spermatogenesis. The thickening of the basal membrane was quite evident and showed extensive infolding. In the Sertoli cells, the presence of spherical or oval membrane-bound granules, vacuoles and degenerating cytoplasmic organelles were observed in the vasectomized group. Degenerative changes of sub-cellular organelles were quite evident in the process of spermatogenesis and spermiogenesis. The number of spermatogonia, spermatocytes, spermatids and mature sperms were considerably reduced; in some cases, almost completely degenerated. Only one type of spermatogonia (Type A) remained intact in the germinal epithelium. Large numbers of multi-nucleate spermatids were observed eight months after vasectomy. These cells were identifiable at all stages of spermatogenesis but were readily detected in cap phase spermatids. There was no evidence of any regeneration occurring in the seminferous tubules examined eight months after vasectomy.

[Development of Golgi apparatus ultrastructure during meiosis and spermiogenesis in the hamster (Mesocricetus auratus)]. / Evolución de la ultraestructura del aparato de Golgi durante la meyosis y la espermiogénesis en el hamster (Mesocricetus auratus).

During meiosis and espermiogenesis the Golgi apparatus shows the greatest morphological and physiological changes. During the second meiotic division it appears as a very large organelle, formed by prominent dictyosomes and by two types of vesicles: one in the external part of the body with diameters that range from 40--100 nm, and the other in the central part of the organelle, larger in size, from 200 to 500 nm. The acrosome, once formed, is spheric (1600 nm in diameter) with the glucoproteins forming a round and dense body occupying its central zone. Later the acrosome moves against the nuclear membrane. These morphological changes occur within a very short time, while the spermatid practically continues occupying the same position in the cellular association of the seminiferous tubule.